A Guide to the Global Positioning System (GPS)

The Parts of GPS

The Space Segment: This part consists of 24 satellites, manufactured by Rockwell International, which are launched into space by rockets, from Cape Canaveral, Florida. They are about the size of a car, and weigh about 19,000lbs. Each satellite is in orbit above the earth at an altitude of 11,000 nautical miles (12,660 miles), and takes 12 hours to orbit one time. There are 6 orbital planes each having 4 satellites. The orbits are tilted to the equator of the earth by 55░ so that there is coverage of the polar regions. The satellites continuously orient themselves to ensure that their solar panels stay pointed towards the sun, and their antennas point toward the earth. Each satellite carries 4 atomic clocks.

The Control Segment: This part consists of 5 worldwide unmanned base-stations that monitor the satellites to track their exact position in space, and to make sure that they are operating correctly. The stations constantly monitor the orbits of the satellites and use very precise radar to check altitude, position and speed. Transmitted to the satellites are ephemeris constants and clock adjustments. The satellites in turn, use these updates in the signals that they send to GPS receivers.

The main base-station is in Colorado Springs, Colorado and the other four are located on Ascension Island (Atlantic Ocean), Diego Garcia (Indian Ocean) and Kwajalein and Hawaii (both Pacific Ocean).

The User Segment:This part consists of user receivers which are hand-held or, can be placed in a vehicle. All GPS receivers have an almanac programmed into their computer, which tells them where each satellite is at any given moment. The GPS receivers detect, decode and process the signals received from the satellites. The receiver is usually used in conjunction with computer software to output the information to the user in the form of a map. As the user does not have to communicate with the satellite there can be unlimited users at one time.